I feel pretty confident that I've seen what the math is for getting XP in a match
2.5 xp/second for a grind (max 25xp)
2.5 xp/second for a manual (max 25xp)
2.5 xp/second for a lip (max 25xp)
Seems as though combo XP is equal to whatever score XP is. Score XP seems to be maxed out at 75 XP when the following two criteria are fulfilled:
Score is at least 250,000
Time of game is at least 2:00
The XP you get is scaled against those two values. If you get 25,000 in 2 minutes, you get (25,000/250,000)*(2:00/2:00)*75XP = 7.5XP (rounded down, so just 7XP)
If you get 10,000,000 in 30 seconds, you get (250,000/250,000)*(:30/2:00)*75XP = 18.5 XP (rounded down to 18).
For those that are curious what testing I've done, I got 240,000 in > 2 minutes and got 70 XP. Then I got 3,000,000 in 80 seconds and got 50 XP.
NOTE: You can boost this with all Game Mods activated. That said, it's still AT BEST 225 XP per 2 minutes, or 6,750 XP per hour.